In hundreds of French "no-go' zones -- neighborhoods where neither tourists nor cops dare enter -- poor and alienated Muslims have intimidated the government into largely ceding authority over them, prompting fears that the kind of jihad that gave rise to last week's attack in Paris is festering unchecked.

In some ways, these 751 areas designated by the French government -- officially called zones urbaines sensibles (sensitive urban zones), or ZUS, for short, but referred to as “no-go” zones by some observers -- resemble poor sections of America’s cities where gangs rule, crime and drugs are rampant a...